252 Mineral Nutrition of Plants 



carry out active accumulation of ions in essentially the same manner 

 as other organs. It has also been verified by several investigators in 

 experiments with tagged cations that the first step of the absorption is 

 reversible and is probably, as was assumed long ago, an adsorption or 

 a tendency toward a Donnan equilibrium on the surface of the plasma 

 colloids. 3) The absorption of anions is of a different nature. It is 

 irreversible and, as has been shown with tagged ions, it takes place not 

 only against the diffusion gradient but also against the charge of the 

 cell (which is predominantly negative) and the adsorption potential. 

 It depends on a portion of the total respiration which is usually called 

 anion respiration and which Robertson has called salt respiration. 4) 

 This anion respiration is different from the rest of the respiration 

 which is called the ground respiration. 



Some of these points, especially point three, the connection of ac- 

 cumulation of anions with respiration, have been questioned, but a 

 number of new data, particularly from Robertson, support the as- 

 sumptions made by Lundegardh. 



New evidence in favor of the opinion that respiration is connected 

 specifically with the intake of anions is found in the determinations of 

 the effect of temperature on ion absorption. I may cite the experiments 

 by Wanner (8). These and other investigations without exception 

 show a significantly higher, sometimes very much higher, temperature 

 coefficient for the absorption of anions than for the absorption of ca- 

 tions. Wanner found a U) of 1.4 for the absorption of potassium from 

 potassium nitrate but a Q U) of about 2 or 2.5 for the absorption of the 

 nitrate ion. With ammonium chloride the corresponding figures were 

 0.9 for the cation and 1.5 for the anion. Still greater differences have 

 been recorded by Jacobson and Overstreet. A low temperature coeffi- 

 cient is characteristic of physical reactions, whereas chemical trans- 

 formations normally have quotients of two or three. These observa- 

 tions, therefore, support the view that the first step in the absorption 

 is a physical phenomenon, while the second step is connected with 

 metabolism, and that this applies especially to the absorption of anions. 



Robertson and his collaborators further have verified the point that 

 respiration is directly connected with the process of ion absorption. 

 Not only does the total respiratory rate increase to a higher level if 



